1. Field of the Invention
This invention relates to calibrating imaging devices to compensate for changes in halftone values during imaging and more particularly to a system for developing a plurality of calibrating curves for such imaging devices using a minimum number of calibrating measurements.
2. Description of the Related Art
Halftone printing and the tools needed in such printing are well known in the art. Halftone printing uses an aggregation of monochromatic dots to produce different shades of gray or other colors. (We will use the term gray scale and black, in describing the present invention, however, as is well understood in the art, the term gray scale designates a series of tonal differences both for black and white as well as color. As used herein, the term black is used to designate a fully exposed area, also referred to as a solid area, while the term white designates a total lack of exposure).
Halftone reproductions rely on the ability of the human eye to integrate a plurality of small black dots on a white background and perceive the dot covered area as a shade of gray. Thus white areas have no dots, or 0% dot coverage, and black areas have 100% dot coverage, that is are fully exposed. Typically the percentage dot coverage of an arbitrarily selected unit area is used to identify its gray level. An area having sufficient dots of sufficient size to cover one half of it is defined as a 50% dot coverage and the dots are 50% dots, and so on.
Two methods are used most often to create the different shades of gray in the gray scale. The first method, known also as AM modulation, and uses black dots of increasing diameters arrayed in a regular matrix along rows and columns at a given frequency, identified as a screen ruling, to create the different gray levels. Screen rulings used in modern day printing typically vary from 50 to 75 lines per inch (LPI) for low quality applications, and from 100 to 200 lines per inch for high quality. What this means is that dots are produced as regular arrays of dots at a centerline pitch of 50/inches or 75/inches. The pitch is often given in micrometers or microns. 50 LPI translates to a distance between adjacent dot centers of 25400 microns/50 or 508 microns. Similarly, a 120 LPI screen ruling places adjacent dot centers at a distance of 196 microns. When the ruling is a 90 degree ruling, each unit area, or cell, containing a single dot has an area of 196.sup.2 microns.sup.2.
Because color is reproduced in halftone reproductions by the superposition of a plurality of monochromatic images of primary colors and therefore by the superposition of arrays of dots, the superposition of different regular arrays of dots can result in Moire patterns which are unacceptable. This problem has been alleviated by aligning the different dot arrays for each monochromatic array at different angles relative to each other. Thus in the reproduction of a colored image there may be involved as many as four or more superposed monochromatic halftone images having dots generated with four or more different screens.
The second method, known as stochastic halftone or FM modulation, uses one size dots at different concentrations per unit area, randomly dispersed, to obtain the same result.
Modern publishing systems use computer driven imaging devices to produce images on an imaging medium with digital signals produced at computer work-stations using application programs that allow reproduction of text and graphic images. In many applications the imaging medium is photosensitive film, and the imaged film is subsequently used to expose therethrough a material useful for making a printing plate for lithographic, flexographic, or other halftone type printing.
In such systems an image appearing as it is desired for it to appear on a final imaging medium, is first created on a computer and displayed on a monitor. Once a proper composition is satisfactory, the different color separation halftones needed to reproduce the image on an imaging medium are created. In creating the halftone color separations, gray levels, from 0-100%, are assigned to different portions of the image in each color separation. The type of halftone method used, i.e. AM or FM modulation, and the halftone screen grid ruling (frequency) and angle must also be specified prior to the computer image being sent to the imaging device.
This information is used by the computer software to create and locate the necessary dot sizes for each color separation. Each color separation is viewed as a separate image having designated gray scale value areas. In the case of AM modulation given a particular screen ruling, a dot pattern is calculated representing the necessary dot sizes to be used for every image area. Depending on the imaging device configuration, this information is usually provided to the imaging device to control the exposing source as the source is scanned across the imaging medium, turning the source on/off to generate an appropriate size dot pixel by pixel.
In printing industry applications, the imaging device is an image setter for exposing, typically, a high contrast silver halide film to produce a color separation with the requested gray values.
In generating each of the color separations, the value of every halftone dot, and thus the reproduced image, is affected by the optics of the imaging device, by the intensity of exposure given the output media, by the photosensitive characteristics of the output media employed, and by chemical processing conditions. The effects of these so-called image processing variables are different for different materials, processes, gray levels, and for different grid screen rulings.
Calibration methods have been developed to compensate for the effects of these different factors on the final image dots so that the gray levels produced on the output media is equal to the gray levels requested by the computer workstation. As currently practiced, these methods typically require direct measurement of a multitude of output test areas containing different size dots, generated during calibration procedures in which a series of gray values are requested as original input for each of every halftone grid screens used and for each of the different output media used. The data obtained from these measurements are typically preserved in a computer memory as a look up table (LUT) or curves in hard copy format in the form of output dot size as a function of requested, or input dot size. From these values or curves one may work backward to obtain correction values or curves that can be applied to the requested dot sizes before such values are used by the image setter, to modify such dot sizes so that the output dot sizes accurately represent the requested sizes. Hereinafter, the obtained correction values, whether stored as LUT or curves are both referred to as "calibration curves".
Specifically, to determine a calibration curve for a single halftone grid screen, a plurality of requested dot sizes that very from 0% to 100% in predetermined increments are sent to an image setter from a computer, and imaged without modification onto the output media. The output media is developed into a final hard copy generally through chemical processes. The gray scale values produced on the imaging media are measured as a function of requested or input gray scale values to obtain a calibration curve for all values requested. (See FIG. 1, for example) Based on these curves, a request for an output 30% dot size if uncorrected will result in a 10% dot. Thus a correction factor is calculated from the calibration curve II in FIG. 1 in effect changing the requested value from 30% to 55%. Requesting a 55% every time a 30% dot is desired results in producing with the particular imagesetter, screen ruling and imaging medium output dots equal to 30% dots.
The procedure to generate this calibration curve is repeated for each halftone grid screen that might be used and then for each different imaging medium that might be used with a particular imaging device. It is normal to use at least 10 different image input gray levels for each different screen and for each different imaging media during this calibration.
Recently, a method using measurements of two rasters having the same selected gray scale value have been used in an effort to calibrate an imagesetter. See for instance U.S. Pat. No. 5,473,734 issued Dec. 5, 1995 to Maskell et al. In this patent Maskell et al. teach using density change measurements between the reproduced gray scale and the requested gray scale to develop a function that permits estimating the change in area coverage for all gray scale values and thus calibrate the imagesetter.
While the Maskell et al calibration method offers a great advantage over the previous method requiring multiple measurements of multiple density scales, it suffers, however, in one respect. Maskell et al assumes in his calculations that dot change during the dot reproduction will be uniform for all size dots because each dot is constructed by same size pixels. This assumption is only partially correct. Individual pixels forming very small and very large dots do not grow by the same amount but are subject to different growth rates primarily due to the photochemistry of the imaging medium and the fact that multiple adjacent pixels forming larger dots in effect receive longer exposures due to the fact that a laser exposing beam has a gaussian distribution which results in higher exposure levels due to overlap at the junction of adjacent pixels.
Maskel's approach provides a close approximation for predicting dot growth particularly with mid size dots. The method becomes progressively less accurate at the two dot size extremes. In addition implementation of the Maskell method requires a process that is conceptually different from the typical calibration process to which the average printer is familiar, requiring a totally different procedure. It is therefore still desirable to develop a calibration method that is both familiar to the operators of typesetting operators and provides good accuracy for all dot sizes and screens, with a minimum number of exposures and measurements.